384 



SCIENCE. 



[N. S. Vol. VIII. No. 195. 



difficult, but meaningless, since the neces- 

 sity of scattering a brood of gametophytes, 

 to avoid competition, has disappeared. It 

 is further true that the development of such 

 a spore involves nutritive supplies from 

 numerous neighboring cells, and a certain 

 amount of retention becomes necessary for 

 this reason. Still further, the advantage to 

 a single megaspore in being retained, thus 

 securing more abundant outside nutrition 

 during germination, would fix the habit if 

 any selective process were at work. For 

 these various reasons it would seem evident 

 that when the sterilization of a megaspo- 

 rangium had reached its extreme limit, by 

 organizing a single spore, retention is likely 

 to follow sooner or later. If this line of rea- 

 soning be true the seed habit might have 

 been developed in any heterosporous line. 

 With the retention of the megaspore pol- 

 lination became necessary, but its gymno- 

 sperm expression differs in no way from the 

 scattering of aerial spores in all the lower 

 groups. The new feature demanded by the 

 retention of the megaspore, therefore, was 

 not the scattering of the microspores, but 

 the development of siphonogamy. That the 

 first retained megaspores were exposed to 

 the microspores can hardly be doubted, and 

 in such cases we now know that the sper- 

 matozoid habit must have been retained, 

 and that no tube, or a very small protuber- 

 ance of the antheridium wall, was needed 

 to discharge the spermatozoids sufficiently 

 near the oosphere. If chemotropism can 

 explain the guidance of a pollen tube 

 through much intervening tissue it would 

 certainly be sufficient to cause the protrusion 

 of an elastic antheridial wall. In the verj^ 

 few illustrations of Cordaites obtained, the 

 megaspore is but slightly covered by sterile 

 tissue at the bottom of a deep pollen cham- 

 ber, and a very slight development of tube 

 is necessary. The same condition is con- 

 tinued in the cycads, and thus the habit of 

 siphonogamyj may have been gradually 



built up. As siphonogamy developed, the 

 gradual failure of the sperm mother cells to 

 organize spermatozoids followed, and pres- 

 ently, almost exclusively now in gymno- 

 sperms, sperm mother cells are found to 

 function directly as male gametes without 

 further organization. 



The secondary results which followed the 

 retention of the megaspore were numerous. 

 The well-known effect of fertilization upon 

 adjacent tissues necessarily involved at least 

 the sporangium, and the seed resulted. The 

 presence of abundant available nutrition 

 and favorable conditions induced the imme- 

 diate germination of the oospore, which the 

 development of a resistant tissue about the 

 sporangium checked. As a consequence, 

 the development of the embryo was thrown 

 into two stages, the intra-seminal and the 

 extra-seminal. 



In the case of the angiosperms, however, 

 another tendency was connected with the 

 retention of the megaspore, namely, the 

 tendency of the sporophyll to enclose the 

 megasporangium, a tendency so evident in 

 such pteridophytes as Isoetes and Marsilea 

 that the direct pteridophyte origin of the 

 group seems more natural than an origin 

 from so specialized a type as the gymno- 

 sperms. Given the reduction of spore pro- 

 duction to a single megaspore and the per- 

 sistent enclosure of the sporangium by the 

 sporophyll, and the angiosperm peculiarities 

 follow. The profound efiect of these con- 

 ditions upon the germination of the mega- 

 spore is so remarkable, and the intergrad- 

 ing stages are so completely unknown, that 

 there seems to be no clue to the sequence 

 of changes. That an endosporic gameto- 

 phyte might eliminate the archegonium 

 seems evident, for the tendency is shown 

 among gymnosperms by Gnetum, where 

 oospheres are organized by free endosperm 

 cells. That the reproductive region of the 

 female gametophyte may be organized 

 earlier than the nutritive region, when the 



